Conventional switching circuit for earphone and loudspeaker of portable information device is shown in FIG. 1. When a ear phone plug is not inserted into the ear phone jack 1, the audio output Lout is passed through the filter 2 and the switch 11 in the ear phone jack 1 to be fed into the operational amplifier 3 for operating the loudspeaker 4. When the ear phone plug is inserted into the ear phone jack 1 to open the switch 11, the audio output Lout is not fed to the loudspeaker 4, but instead to an earphone.
The operational amplifier 3 is designed into an integrated circuit, while the capacitor 21, the resistor 22 in filter 2 and the input bias resistors 31, 32 of the operational amplifier 3 are still designed by passive components, and arranged outside the IC of the operational amplifier 3. It is tedious to assemble the passive components in this kind of design, and the passive components occupy too much area in a circuit board, therefore increase the cost and volume, and are not suitable in designing a compact portable information device.